There is known an engine provided with a variable water pump which can voluntarily drive and stop a water pump. In such an engine, the circulation of cooling water is stopped in warming up the engine, thereby warming up the engine early, and thereby improving fuel consumption and emission. Although an electric water pump has been already employed as the variable water pump, the electric water pump has a high cost, and requires high power to need a battery with a high capacity.
In addition to the electric water pump, there is known another variable water pump of an electromagnetic clutch type. In such another variable water pump, an electromagnetic clutch engages or disengages a drive shaft of the engine with a drive shaft of the water pump so as to vary an amount of the cooling water pressure-fed. This variable water pump of the electromagnetic clutch type has a cost lower than that of the electric water pump, and has attracted attention as an alternative technology of the electric water pump.
Patent Document 1 describes an improved variable water pump of the electromagnetic clutch type. In the water pump described in Patent Document 1, a coil of the electromagnetic clutch is energized in a lower load state to reduce a cooling water amount supplied by the water pump.